The present invention relates to a pressure equilibrator for providing a constant difference or ratio of the pressures of two gases which flow continuously. Although the application of the pressure equilibrator is not particularly specified, it is suitable for use in a gas mixing apparatus of a hemataerometer.
In a hemataerometer, at least two kinds of aqueous solutions, in each of which the partial pressures of carbon dioxide gas and oxygen gas are known, are used as reference liquids for the purpose of comparison and correction. A reference liquid is in general prepared by saturating water with a known mixture of carbon dioxide gas, oxygen gas, and nitrogen gas, which is charged in a cylinder. For this purpose, a plurality of gas mixtures have to be prepared beforehand for the reference liquids.
U.S. Pat. No. 3,464,434 discloses, in particular in FIGS. 4 and 5 thereof, an improved gas mixing apparatus. This gas mixing apparatus is adapted to mix gases by controlling the flow rates of supplied gases by means of resistance tubes, and is capable of producing a mixture containing oxygen gas and nitrogen gas by mixing air with pure carbon dioxide gas. The mixing apparatus is provided with a differential pressure control valve of the relief-valve type so as to keep the pressure of the air constant with respect to that of the carbon dioxide gas.
With this gas mixing apparatus, because of the characteristics of the resistance tubes, if, for instance, the apparatus is used in a hemataerometer and carbon dioxide gas is to be supplied from a cylinder, the pressure of the carbon dioxide gas must be once reduced from a pressure of the gas within the cylinder of about 65 kgf/cm.sup.2 at room temperature to a pressure of about 2 kgf/cm.sup.2 by a pressure reduction valve and be further reduced therefrom to a pressure of about 0.1 kgf.multidot.cm.sup.2 at which point the gas has a stability of not more than 1%. This means that, since an ordinary commercially-available pressure reduction valve is unable to achieve such a reduction in pressure, the apparatus has to be provided with a special pressure reduction means.
The aforesaid differential pressure control valve is constructed as a combination of a diaphragm and a relief port, and is adapted to introduce pressurized air into one side of the diaphragm which is closer to the relief port and introduce carbon dioxide gas under a constant pressure into the other side, so that, if the pressure of the air becomes higher than that of the carbon dioxide gas, the diaphragm moves to open the relief port and allow air to be discharged, thereby keeping the difference in pressure between the two gases constant. With this arrangement, however, the rigidity of the diaphragm has to be high enough to be able to keep the relief port tightly closed, and this, in turn, requires a large pressure receiving area for driving the diaphragm. These requirements make it impossible for the differential pressure control valve to provide sufficient control over relatively small fluctuations in gas pressure. A fatal drawback of the control valve is that it is constructed with no regard to fluctuations in the pressure of carbon dioxide gas. This means that, if the pressure of the carbon dioxide gas becomes excessive, the control valve stops functioning.
Japanese Patent Unexamined Publication No. 59-110968 discloses a control apparatus which is capable of responding to fluctuations in the pressures of two fluids. The control apparatus has a cylindrical housing, a piston-like member which is movably disposed within the housing to form two opposing chambers within the housing, and a relief port disposed in a portion of the housing between the two chambers. With this apparatus, two fluids are introduced into the respective chambers, and the piston-like member moves in response to the difference in pressure between the two fluids, whereby the pressures of the fluids are equilibrated. Additionally, if the pressure of one of the fluids becomes excessive, the piston-like member moves through a stroke long enough that the chamber into which that fluid is introduced communicates with the relief port, thereby discharging the excess pressure. This apparatus is, however, disadvantageous in that, since the piston-like member is disposed in a fluid-tight manner so as to prevent the two fluids from mixing with each other, the piston-like member is subjected to large friction resistance. Consequently, the apparatus does not operate in response to small differential pressures of about 0.001 to 0.02 kgf/cm.sup.2.